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The Air car is powered by an air engine, specifically tailored for the car. The engine is powered by compressed air, stored in a carbon-fiber tank at 4500 psi.The tank being made of carbon-fiber in order to reduce its weight. The engine has injection similar to normal engines, but uses special crankshafts and pistons, which remain at top dead center for about 70% of the engine's cycle; this allows more power to be developed in the engine.
The French version
Engineers in France believe they have come up with the answer that environmentalists and economists have spent years searching for: a commercially viable, non-polluting car, which costs next to nothing to run.
The latest prototype will be unveiled on Thursday at the Paris motor show.
Like everything else about this vehicle, it all sounds impossible.
When we went to the company's factory-cum-design shop just outside Nice in the South of France a black blanket was put between us and it.
But we were told it has a steering wheel in the middle, with passenger seats either side, a boot the size of the biggest estate, but in overall size terms is no bigger than a Smart car.
The air is compressed at pressure about 150 times the rate you would put into car tyres or your bicycle.
An earlier version of the car that we drove was noisy and slow, and a tiny bit cumbersome.
But then this vehicle will not be competing with a Ferrari or Rolls Royce. And the manufacturers are not seeking to develop a Formula One version of the vehicle.
What the company is aiming at is the urban motorist: delivery vehicles, taxi drivers, and people who just use their car to nip out to the shops.
The latest vehicle is said to have come on leaps and bounds from the early model we drove.
It is said to be much quieter, a top speed of 110 km/h (65 mph), and a range of around 200 km before you need to fill the tanks up with air.
Filling up
The car comes fitted with its own compressor so you can fill up at home. But that would take four hours.
The company has developed the technology to refill the vehicle in three minutes, although there are no service station forecourts with the compressed air machines to do that yet.
And the cost? Cyril Negre, the head of Research and Development at MDI cars, reckons a full tank of air would be about 1.50 euros.
But the difference between success and failure in the motor industry is investment and faith. On paper the car works; around the industrial estate that we took the early prototype, it works.
Now the question is: how to make the leap from concept to the market? Will the dream become something more than so much compressed air?
Engine
--Two Cylinder Air-Compression Engine
The e.Volution is powered by a two-cylinder, compressed-air engine. The basic concept behind the engine is unique. It can run either on compressed air alone or act as an internal combustion engine. Compressed air is stored in carbon or glass fiber tanks at a pressure of 4,351 pounds per square inch (psi). This air is fed through an air injector to the engine and flows into a small chamber, which expands the air. The air pushing down on the pistons moves the crankshaft, which gives the vehicle power.
--Cryogenic Heat Engine
Another version of an air-powered car is being developed by researchers at the 'University of Washington' using the concept of a steam engine, except there is no combustion. The Washington researchers use liquid nitrogen as the propellant for their LN2000 prototype air car. The researchers decided to use nitrogen because of its abundance in the atmosphere -- nitrogen makes up about 78 percent of the Earth's atmosphere -- and the availablity of liquid nitrogen. There are five components to the LN2000 engine:
a)A 24-gallon stainless steel tank. 
b)A pump that moves the liquid nitrogen to the economizer.
c)An economizer that heats the liquid nitrogen with leftover exhaust heat.
d)A heat exchanger that boils the liquid nitrogen, creating a high pressure gas.
e)An expander, which converts nitrogen's energy into usable power.
The liquid nitrogen, stored at -320 degrees Fahrenheit (-196 degrees Celsius), is vaporized by the heat exchanger. The heat exchanger is the heart of the LN2000's cryogenic engine, which gets its name from the extremely cold temperature at which the liquid nitrogen is stored. Air moving around the vehicle is used to heat the liquid nitrogen to a boil. Once the liquid nitrogen boils, it turns to gas in the same way that heated water forms steam in a steam engine. 
Nitrogen gas formed in the heat exchanger expands to about 700 times the volume of its liquid form. This highly pressurized gas is then fed to the expander, where the force of the nitrogen gas is converted into mechanical power by pushing on the engine's pistons. The only exhaust is nitrogen, and since nitrogen is a major part of the atmosphere, the car gives off little pollution. However, the cars may not reduce pollution as much as you think. While no pollution exits the car, the pollution may be shifted to another location. As with the e.Volution car, the LN2000 requires electricity to compress the air. That use of electricity means there is some amount of pollution produced somewhere else.
Some of the leftover heat in the engine's exhaust is cycled back through the engine to the economizer, which preheats the nitrogen before it enters the heat exchanger, increasing efficiency. Two fans at the rear of the vehicle draw in air through the heat exchanger to enhance the transfer of heat to the liquid nitrogen.
Similar cars in development
The Energine Corporation is a company that delivers fully assembled cars running on a hybrid compressed air and electric engine: these cars are more precisely named pneumatic-hybrid electric vehicles(PHEV).
A similar (but only for braking energy recovery) concept using a pneumatic accumulator in a largely hydraulic system has been developed by U.S. government research laboratories and may soon be produced for delivery van use.
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the compressed air technology engine working underwater
